Proteasomes are large ring- or cylinder-shaped multicomponent complexes common to all eukaryotic cells (Tanaka et al (1995) New Biol 4: 173-187). They have at least three distinct endopeptidase activities which include hydrolysis of peptide bonds on the carboxyl side of hydrophobic, basic, and acidic amino acid residues (Rivett A J (1993) Biochem J 291: 1-10). Proteasomes, through their protein degradation activity, have been implicated in several important cell functions, including DNA repair, cell cycle progression, signal transduction, transcription, and antigen presentation (Finley D et al (1991) Annu Rev Cell Biol 7: 25-69).
Proteasome subunits are encoded by a family of homologous genes. The 20S proteasome consists of a family of 14 different subunits that are classified into 7 different but homologous .alpha.-type or B-type subunits (Yang Y et al (1995) J Biol Chem 270: 27687-27694). Changes in the composition of the 20S proteasome are correlated with changes in substrate specificity. Three B-type subunits, LMP2, LMP7, and MECL-1 are upregulated by interferon gamma, and replace their constitutive counterparts, delta and MB-1, in the complex (Larsen F et al (1993) Hum Mol Genet 2: 1589-1595); Martinez CK et al (1991) Nature 353: 664-667; Akiyama K et al (1994) Science 265: 1231-1234). It has been suggested that incorporation of these subunits into the proteasome may generate so called "immunoproteasomes` that could represent a proteasomal subpopulation capable of more efficiently processing protein antigens into the short peptides that are transferred to the class I major histocompatibility complex (MHC) for presentation on the cell surface to cytotoxic lymphocytes (Tanaka K (1994) J Leuk Biol 56: 571-575). An example of the .alpha.-type subfamily of proteasome subunits, RC6-I, cloned from rat cells by Ni R et al (1995, Biochim Biophys Acta 1264: 45-52), is expressed in all the rat tissues examined, yet may also contribute to the diversity of proteasome functions.
Proteasomes and Disease
A decrease in muscle mass, known as muscle wasting or cachexia, has been shown to be associated with the proteasome-dependent proteolytic system. Rats bearing the Yoshida AH-130 ascites hepatoma for 7 days showed a significant decrease in muscle mass in relation to non-tumor bearing controls (Llovera M et al (1995) Int J Cancer 61: 138-141). The muscle wasting was found to be associated with an increased proteolytic rate related to the proteasome-dependent proteolytic system. Muscle wasting is common among human cancer patients. In addition to cancer, proteasome-dependent muscle wasting is also influenced by nutritional manipulation (such as fasting and dietary protein deficiency), muscle activity and disuse, AIDS, and the pathological conditions, sepsis, trauma; and acidosis (Attaix D et al (1994) Reprod Nutr Dev 34: 583-597).
The presence of proteasome-dependent proteolysis has been detected in patients affected by neurodegenerative diseases such as Alzheimer's disease. Whereas the intracellular amyloid beta-protein precursor (APP) did not show appreciable proteasome-mediated degradation, three extracellular APP forms were degraded by this proteolytic pathway, suggesting a potential regulatory role for the proteasome-mediated system in the in vivo APP metabolic pathway (Gregori L et al (1994) Biochem Biophys Res Commun 203: 1731-1738). Paired helical filaments (PHF) are fibrillar structures that accumulate in degenerating neurons in the brains of Alzheimer's disease patients. One component of PHF, the PHF-smear, consists of the tau protein fragment tagged for proteasomal degradation (Morishima M et al (1994) Dementia 5: 282-288).
Evidence from experiments on mouse and rabbit reticulocytes indicate that proteasome degradation is a key rate-limiting step in antigen presentation (Grant EP et al (1995) J Immunol 155: 3750-3758). The rates of degradation of beta-galactosidase constructs correlated with the rates of class I antigen presentation in vivo. This shows that proteasome degradation pathway may have a critical role in generating MHC class I-presented peptides.
Depletion of specific cellular proteins may have many medical and agricultural benefits. Redirecting the proteasome-dependent proteolytic pathway may facilitate specific protein removal. Thus, it may be possible to design proteasome subunits capable of directing the selective removal of many intracellular proteins, such as those implicated in the pathogenesis of Alzheimer's disease. The selective modulation of proteasome activity may allow successful management of the diseases associated with protein degradation, such as muscle wasting syndrome, and diseases in which undesired proteins may be targeted for degradation, such as viral infections and cancer. A newly discovered proteasome subunit may have novel specificity and could thus target a unique set of proteins for degradation.
More than a million Americans suffer from dementia, a permanent and often progressive decline in intellectual function that substantially interferes with a person's social and economic activity. Alzheimer's disease is a major cause of dementia and its prevalence is growing. Currently, there are no known treatments that stop or reverse the relentless progression in the impairment of mental abilities of patients afflicted with Alzheimer's disease. Similarly, there are no known treatments that fully reverse muscle wasting, a disorder common among a growing number of AIDS patients. New proteasome subunit proteins could satisfy a need in the art by providing new means of diagnosing and treating Alzheimer's disease and muscle wasting syndrome.